Various industries, and particularly the manufacturing industry, among others, have utilized rotary motors and linear actuators to control movements of automated welding guns, automated clamping fixtures, and the like. For example, in the automotive industry, injection molding industry, and various other industries, actuation and control of welding guns and clamping fixtures and controlled linear movement of other fixtures and devices have been accomplished using fluid actuators, such as pneumatic or hydraulic actuators. While fluid actuators have functioned reasonably well for these purposes, they inherently embody various limitations. One, because of the possibility of leaks and failure of seals, etc., there is always the concern of contamination of the worksite by a leaking fluid. Second, fluid actuators necessarily require a source of pressurized fluid, and thus, a fluid supply system. This leads to significant maintenance and other costs. Third, limitations sometimes exist with respect to the accuracy and positioning of linear movement and the adjustability of such movement.
The use of permanent magnet, brushless motors is also well known. A permanent magnet, brushless motor is described in co-pending U.S. patent application Ser. No. 11/031,539, filed Jan. 7, 2005, entitled “Electric Actuator,” and published as Publication No. 2005/0253469, the entirety of which is hereby incorporated by reference herein. The relationship between the rotation and torque of prior art permanent magnet, brushless motors is inversely proportional. That is, as the torque linearly decreases, the rotation speed, or number of rotations, increases.
In some prior art permanent magnet, brushless motors, a field weakening technique wherein the total magnetic flux is lowered to achieve high speed rotation has been employed. For example, a brushless motor that includes a field weakening technique is described in U.S. Pat. No. 5,821,710, issued to Masuzawa, et al. The brushless motor in Masuzawa includes two field permanent magnets having poles of different polarities alternately arranged in the direction of rotation, wherein one of the field permanent magnets is rotatable with respect to the other field permanent magnet. A mechanism for changing the phase of the magnetic poles of the field permanent magnets is provided to place the field permanent magnets out of phase as rotation increases. The mechanism uses arc-shaped governors held in a default, low rotation position using springs. The governors are forced into a high rotation position due to centrifugal force caused by the higher speed rotation. The high rotation position causes the field permanent magnets to be positioned out of phase, thus weakening the magnetic field.
Accordingly, there is a need in the art for improved apparatus and methods for a permanent magnet, brushless motor having field weakening capability which overcomes the deficiencies and limitations of the prior art. Particularly, there is a need in the art for apparatus and methods for a permanent magnet, brushless motor that may automatically transition from a field weakened position upon encountering a significant load.